1. Field of the Invention
The present invention relates generally to a photomask for use in a projection exposure method, and more particularly to a phase shift mask for use in fabricating a device with a fine structure, such as a semiconductor device or a liquid crystal display device.
2. Description of the Related Art
In a semiconductor device fabrication process or a liquid crystal display device fabrication process, photolithography techniques are generally employed. In the photolithography, a photoresist is coated on a substrate on which a semiconductor device or a liquid crystal display device is to be formed, or on a thin film that is formed on the substrate. The photoresist is selectively exposed, thus forming a mask for microfabrication, such as an etching mask. Using the etching mask, the substrate or the thin film on the substrate is subjected to a selective etching process, etc.
A photomask for exposure is used in order to perform selective exposure of the photoresist. A conventional photomask for exposure is formed, for example, by providing a light-shield pattern on a transparent substrate. Thereby, the intensity of transmissive light is amplitude-modulated. Typically, the photomask for exposure is configured such that a light-shield pattern of a metal thin film of, e.g. chromium is formed on a transparent substrate. Light is radiated to pass through the photomask, and an image of a light-shield pattern is focused on the substrate by a projection optical system. Thus, a desired circuit pattern is transferred to the substrate.
Further, in order to enhance the contrast of a fine projected image, various kinds of phase shift masks have recently been used, which are configured such that phase shift portions for varying the phase of transmissive light are provided at specific locations on transparent portions. An example of such a phase shift mask is a Shibuya-Levenson type phase shift mask. In the phase shift mask, transmissive regions for forming fine projected pattern portions are provided with phase shifter regions which provide a phase difference of 180° between the transmissive regions. When light from a light source (generally, unpolarized light including various polarization modes) is radiated on the mask, a clear dark part is formed at an overlapping part of light components that pass through both phase shifter regions. The reason is that the same polarized light components interfere with each other with opposite phases.
As a result, a light-shield region that is interposed between both phase shifter regions, or a projected pattern portion indicating the boundary between both phase shifter regions, is clearly formed. Hence, the resolution of the fine projected pattern image can be enhanced, or the focal depth can be increased.
As has been described above, in the field of microfabrication of, e.g. a semiconductor device fabrication process or a liquid crystal display device fabrication process, the amplitude-modulation type photomask that generates the above-described amplitude distribution or the phase shift mask that also generates the phase distribution is used as the photomask for use in exposure of the resist.
In order to form a finer pattern, a “polarized phase shift mask”, which has a function of polarizing radiation light, in addition to the above-described functions, has recently been proposed. In the polarized phase shift mask, regions that create different polarization states are formed with a predetermined distribution.
The purpose of use of the polarized phase shift mask is to solve the problem of “phase conflict”, which is a problem with conventional phase shift masks (limited to a Shibuya-Levenson mask or an alternating phase shift mask in the present case). Prior-art documents, which describe the phase conflict, include Ruoping Wang et al., Polarized Phase Shift Mask: Concept, Design, and Potential Advantages to Photolithography Process and Physical Design, Proceedings of SPIE Vol. 4562, 406-417 (2002), Jpn. Pat. Appln. KOKAI Publication No. 2002-116528, and Jpn. Pat. Appln. KOKAI Publication No. 5-11434. A general description of the phase conflict will be given below.
FIG. 1 shows an example of a phase shift mask (Shibuya-Levenson mask) 100. FIG. 1 shows an exposure pattern in a case where a phase shift function is to be applied to two isolated linear patterns 103 and 104. Dark parts (indicated in black) are light-shield parts 101 which become non-exposed parts. In the case of using the phase shift mask (Shibuya-Levenson mask), it is necessary to provide a phase difference of 180° between both sides of the light-shield part 101, which is a fine line that is an object of microfabrication.
In the structure shown in FIG. 1, however, a boundary 102, which has a phase difference of 180°, is also formed at a location other than the location of the light-shield parts 101. The part of the boundary 102 becomes a non-exposed part since the light intensity at this part becomes zero due to interference. Consequently, in the case of using a positive-type resist in which an exposed part is removed, a corresponding resist part is not removed and remains. Conversely, in the case of using a negative-type resist, a corresponding resist part is removed (in the description below, a positive-type resist is used).
In order to solve this problem, a so-called “polarized phase shift mask” 110, as shown in FIG. 2, has been developed. The polarized phase shift mask is configured such that a function of a polarized mask is added to the conventional phase shift mask 100 shown in FIG. 1, thereby to solve the problem of phase conflict.
In FIG. 2, arrows 111 and 112 indicate directions of linear polarizers provided on the mask, that is, directions of linearly polarized light that has just passed through the mask. No interference occurs between light rays in different polarization directions. Hence, even if there is a phase difference at a boundary 113, the intensity of the transmissive light does not become zero, and there arises no such problem that a corresponding resist part is left (however, this problem is not completely solved, as will be described later).
In order to realize the above-described function, a fine patterning process for forming a structure with a polarizing function on a photomask is indispensable in the formation of the polarized phase shift mask. As specific methods which realize formation of the polarizing function, methods of providing electrical conductor lattices, for example, are disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 9-120154, 7-36174 and 7-176476.
FIG. 3A to FIG. 3C show a mask 120 which is provided with an electrical conductor lattice 121 of, e.g. chromium, in order to constitute a polarizing function. The electrical conductor lattice 121 is generally called “grid polarizer”. As shown in FIG. 3A, for example, the electrical conductor lattice 121 is a lattice that is formed of a plurality of conductors 122 with a lattice pitch less than the wavelength of exposure light.
The electrical conductor lattice 121 shown in FIG. 3A has such a feature that light 123 with a direction of electric field that is perpendicular to the lattice is transmitted (124), as shown in FIG. 3B, and light 125 with a direction of electric field that is parallel to the lattice is reflected (126), as shown in FIG. 3C. Accordingly, the electrical conductor lattice 121 is usable as a linear polarizer. With the electrical conductor lattice 121, the function of the polarization mask can be realized.
The electrical conductor lattice 121 can be fabricated by an ordinary lithography method which is used, for example, in a semiconductor device manufacturing process in which microfabrication can be performed. Specifically, a chromium thin film, for instance, is formed on a silica substrate 127 by sputtering, following which resist coating, patterning and etching are performed to fabricate the electrical conductor lattice 121.
Aside from the grid polarizer, a photonic crystal is known as a material of a polarization modulation element which realizes the function of the linear polarizer. In addition, a material having birefringence effect is usable for the polarization modulation element. A structural birefringence element or a photonic crystal is known as achieving birefringence effect.
Next, a description is given of the problem in the case of using the polarized phase shift mask as a photomask, as shown in FIG. 2. In the above description of the prior art, it is stated that “No interference occurs between light rays in different polarization directions. Hence, even if there is a phase difference at a boundary 113, the intensity of the transmissive light does not become zero, and there arises no such problem that a corresponding resist part is left”. It has been understood, however, that this effect is incomplete.
FIG. 4 shows a cross-sectional view (a) of a polarized phase shift mask 130, and a calculation result (b) of a light intensity distribution of transmissive light in a part where a phase conflict due to the polarized phase shift mask is to be eliminated. In FIG. 4, the polarized phase shift mask 130 is configured such that two polarization modulation regions 131 and 132 are formed on a transparent substrate 137, and two phase modulation regions 133 and 134 are formed on the polarization modulation regions 131 and 132. The conditions for an optical system are set as follows: wavelength=365 nm, NA (numerical aperture)=0.25, and σ=(coherence factor)=0.4. All calculation results below are based on these conditions.
The inventors analyzed transmissive light from the polarized phase shift mask 130 shown in part (a) of FIG. 4. It was found that the light intensity distribution of the transmissive light does not have an ideal flat characteristic, but has such a characteristic as shown in part (b) of FIG. 4.
In part (b) of FIG. 4, a curve A indicates the intensity of light that has passed through a region A (phase 0°) and a curve B indicates the intensity of light that has passed through a region B (phase 180°). An actual light intensity distribution of the light that has passed through the mask 130 is the sum of both intensities, which is indicated by a curve C in part (b) of FIG. 4. In this patent specification including descriptions below, the light intensity is expressed by a value which is set by normalizing the light intensity in the background part of the mask (i.e. the part which normally passes light) at 1.
According to the calculation result shown in part (b) of FIG. 4, the light intensity at a boundary 135 between the regions A and B is 0.5. This value is higher than a theoretical light intensity of zero, which is based only on a phase shifter without the polarization modulation regions 131 and 132, but it does not become 1 (a part 136 at which the light intensity is minimum is referred to as “dip part”, and the light intensity at the dip part is referred to as “dip intensity”). Owing to the formation of the dip part, if the exposure amount decreases, the exposure amount may, in some cases, fall below the threshold of the resist at the position corresponding to the dip part. In such cases, a resist portion may possibly remain due to deficiency in exposure. To be more specific, compared to the case where the dip intensity is 1, the “exposure amount tolerance”, which realizes formation of a desired pattern, becomes narrower.
Next, an example of the exposure amount tolerance is calculated in brief. In the description below, the exposure amount is defined as follows: (exposure amount)=(intensity of background part)×(exposure time). In addition, a proper exposure amount is normalized at 1, and the threshold (i.e. boundary of dissolution/non-dissolution) of the resist in this case is set at 0.4. Under the conditions in FIG. 4, if the exposure amount decreases only slightly from the proper value 1 to 0.8 (=0.4/0.5) or less, the exposure amount at the boundary 135 falls below the threshold of 0.4. As a result, the resist is not removed and remains at the dip part 136. If the dip part 136 does not occur, there arises no such problem that the resist remains at the exposure amount of 0.4. In other words, the exposure tolerance is narrowed to ⅓(=(1−0.8)/(1−0.4)).
In the above calculation, it is assumed that the polarization modulation layer 131, 132 and the phase modulation layer 133, 134 are provided completely in the same plane. However, in fact, the structure shown in part (a) of FIG. 4, wherein these layers are stacked, is generally adopted. In this case, the dip intensity further decreases due to the effect of Fresnel diffraction occurring between the layers. Consequently, the exposure amount tolerance further decreases.
FIG. 5 shows a case where a gap of 0.2 μm, which is a converted value in air (0.3 μm in a substance with a refractive index of 1.5), is present between the polarization modulation layer 131, 132 and the phase modulation layer 133, 134. FIG. 5 shows a calculation result of the intensity of transmissive light in the case where such a gap is present. In this case, the light intensity at the dip part 136 decreases to 0.3, and the exposure amount tolerance becomes still narrower.
The problem with the phase conflict part, as shown in FIGS. 4 and 5, has been explained above. The above-described dip part inevitably occurs at the boundary between two polarization modulation regions, regardless of the presence/absence of a phase stepped part.